Leuco dyes are colour forming agents that are colourless in one form but can change into a coloured form when exposed to a particular stimulus. Most leuco dyes respond to changes in pH, typically being colourless under alkaline conditions but becoming coloured in an acidic environment. Leuco dyes form the basis of thermal paper technology in which the leuco dye is used in combination with an acid generating species. Upon application of heat the acid generating species yields protons that interact with the leuco dye and convert it into its coloured form. A well known acid generating species is benzyl p-hydroxybenzoate.
However, conventional leuco dye/acid generating species systems have certain disadvantages. They are known by those skilled in the art to give rise to very poor solvent and scratch resistance, and liquid formulations containing these species such as inks are unstable and difficult to use. Thus there is a need for alternative thermal acid generator systems.